The present application relates to and incorporates herein by reference Japanese Patent Applications No. 2000-402417 filed on Dec. 28, 2000 and No. 2001-321948 filed on Oct. 19, 2001.
1. Field of the Invention
The present invention relates to a data receiver for receiving data encoded employing predetermined encoding.
2. Related Art
Some of vehicles which employ pneumatic tires include a tire condition monitoring system for improving running safety. The tire condition monitoring system monitors the internal state of the tires. Specifically, in the tire condition monitoring system, tire condition warning devices incorporated inside the tires detect the air pressure in the tires or an abnormal condition therein, and then the detected information is transmitted to a data receiver installed in the vehicle body via radio waves.
The tire condition warning devices encode information on the tire pressure into binary data, and then carrier waves is modulated using the encoded information and transmitted to the data receiver. Various signal formats for representing binary data is known, and particularly a non-return-to-zero (NRZ) format or a differential bi-phase (DBP) format is known as a typical signal format for representing binary data. According to the NRZ format, each bit (xe2x80x980xe2x80x99 or xe2x80x981xe2x80x99) of binary data is represented by polarity. According to the DBP format, each bit of binary data is represented by presence or absence of a pulse edge synchronized with a reference clock signal, that is, it is represented by the pulse width between two pulse edges synchronized with regular reference clock signals.
The tire condition warning system usually employs DBP format, because each bit of the binary data can be obtained only by detecting pulse edges and pulse widths between two consecutive pulse edges from a pulse string. That is, the binary data can be restored from the pulse string without using clock signals in this case, while it is restored using clock signals in the case of synchronous communication or the like.
The tire condition monitoring system employs the data receiver for receiving the carrier waves modulated using a signal that represents the binary data in the DBP format from the tire condition warning devices. The data receiver detects and demodulates the received signal so that a pulse string which represents the binary data is extracted. Further the pulse edges and the pulse widths between two consecutive pulse edges are sequentially detected from the extracted pulse string, so that each bit of the binary data is restored. Thus the original binary data is restored from the received signal.
However, the data receiver sometimes obtains incorrect binary data as a result of restoration due to external noise or the like incorporated in the signals. The obtained binary data may be considerably incorrect when the condition (level of electromagnetic field) of the space through which the signals are transmitted to the data receiver is relatively poor. This problem is not limited to the transmission of the signals in the DBP format. That is, this problem usually arises when data is transmitted by cables or via radio waves.
Therefore, the data receiver usually eliminates the external noise using a hardware filter during detection and modulation of the received signal, so that components within a required frequency band (e.g., the frequency band of carrier waves which a transmitter modulates using the encoded binary data) are extracted.
Further, if the data receiver detects a pulse edge from the pulse string during the restoration of the binary data, it validates the detected pulse edge using a software twice-match filter. That is, the detected pulse edge is not validated immediately after it is detected. Specifically, the data receiver samples the pulse string according to predetermined reference clock signals. When the pulse edge is detected at a sampling time, the detected pulse edge is validated only if the same level is detected at the next sampling time. Thus, if an erroneous pulse whose width is shorter than the sampling interval is incorporated in the pulse string due to noise or the like, it is invalidated by the twice-match filter. As a result, the precision of the decoding is improved.
However, external noise within the required frequency band is not eliminated by the hardware filter. If the intensity of the external noise is relatively high, the noise is incorporated in the pulse string as an erroneous pulse.
If the erroneous pulse has a pulse width shorter than the sampling interval, it is eliminated by the twice-match filter even when it passes through the hardware filter. However, the sampling interval usually corresponds to the cycle of the reference clock incorporated in a microcomputer for operating its CPU. The cycle of the reference clock is very short (several microseconds), and therefore the erroneous pulse which has a very short width is only eliminated by the twice-match filter.
That is, an erroneous pulse which is within the required frequency band and has a width longer than the sampling interval is not eliminated, and therefore the edges of such an erroneous pulse is detected and validated. As a result, the original binary data is not accurately restored from the pulse string, and consequently a reception error occurs.
In the data receiver of the above tire condition monitoring system, the pulse string would include many erroneous pulses which cannot be eliminated by the software twice-match filter. The reason is as follows. The tire condition warning devices of the tire condition monitoring system are mounted inside the tires (on external surface of the wheels) as described above. Therefore, when the vehicle travels, the tire condition warning devices rotate as the tires rotate. On the other hand, the data receiver is fixed to the vehicle body, and therefore the level of the signals received by the data receiver constantly varies.
Particularly, the data receiver cannot receive the signals from the transmitter of the tire condition warning devices during a period (non-receipt period) whenever the tires make one rotation, because the level of the signals are low during the period due to an adverse effect of the directivity of antennas of the transmitter and the receiver. The null angle, which is a mechanical angle of the tires corresponding to the non-receipt period, varies depending on physical relationship between the antennas of the transmitter and the receiver, the power of the transmitted signals and the like. In the case of the tire condition monitoring system, it is known as a result of trail measurement that the null angle is several degrees.
When a vehicle, which has tires whose periphery is 2 meters long, travels at a speed of 100 km/h (low speed), the level of signals received by the data receiver varies as shown in FIG. 6A as the tires rotate. In this case, the tires make one rotation in 72 ms. Assuming the null angle is 2 degrees, the data receiver can receive signals during 71.6 ms (T1: receipt period) out of 72 ms, while it cannot receive signals during 400 xcexcs (Tf1: non-receipt period) out of 72 ms. That is, the data receiver cannot receive signals during the non-receipt period Tf1 corresponding to the null angle whenever the tires make one rotation.
When the same vehicle travels at a speed of 300 km/h (high speed), the level of signals received by the data receiver varies as shown in FIG. 6B as the tires rotate. In this case, similarly to the case of the low-speed travel, the data receiver cannot receive signals during a period whenever the tires make one rotation. However, a receipt period T2 is 23.867 ms, and the non-receipt period Tf2 is 133 xcexcs That is, both of the receipt period T2 and the non-receipt period Tf2 are shorter than the case of the low-speed travel, because the tires rotate at a high speed in this case.
Assuming that time required for receiving one frame of the binary data is 64 ms, the receipt of one frame can be completed within the receipt period T1 when the vehicle travels at the low speed. Further the data receiver repeatedly receives the same data several times, and obtains the correct data from the received data by majority. Therefore there is little possibility that the data receiver cannot obtain the correct data in the case of the low-speed travel, even if the data is received during a time including the non-receipt period Tf1 once or twice out of the several times.
However, in the case of the high-speed travel, the receipt of one frame cannot be completed within the receipt period T2. That is, the non-receipt period Tf2 absolutely comes during the receipt of one frame. The non-receipt period Tf2 is reflected in the pulse string as an erroneous pulse. The erroneous pulse cannot be eliminated by the software twice-match filter, because the width of the erroneous pulse is 133 xcexcs, that is, longer than the sampling interval. As a result, the edge of the erroneous pulse is detected and validated, and consequently a reception error occurs.
The receipt period T1, T2 decreases as the traveling speed of the vehicle increases as described above. Accordingly, in order that the data receiver receives one frame within the receipt period when the vehicle travels at a high speed, the communication speed should be increased or the number of bits included in one frame should be decreased.
However, the reception sensitivity decreases as the communication speed increases. Further the number of bits included in one frame is necessarily determined depending on the number of bits at least required for representing information (e.g., tire pressure and temperature) communicated in the tire condition monitoring system. That is, the communication speed or the number of bits included in the frame is necessarily set to a certain value.
It is therefore an object of the present invention to provide a data receiver capable of accurately restoring binary data from a received signal modulated using a signal in which each bit of the binary data is represented by presence or absence of a pulse edge synchronized with a reference clock signal.
A data receiver according to the present invention includes receiver means, edge detection means, determination means, data restore means, and edge invalidation means. The receiver means receives a pulse string in which each bit of binary data is represented by a pulse width between two consecutive pulse edges synchronized with regular reference clock signals. The edge detection means sequentially detects a pulse edge from the pulse string. When a pulse edge is newly detected by the edge detection means, the determination means determines whether the pulse width between the preceding pulse edge and the newly detected present pulse edge is synchronized with the reference clock signals.
However, if the pulse width between the preceding pulse edge and the present pulse edge is shorter than one cycle of the reference clock signals, the edge invalidation means invalidates the present pulse edge so that the present pulse edge is not inputted to the determination means. When the determination means determines that the pulse width is synchronized with the reference clock signals, the data restore means determines next one bit of the binary data based on the pulse width. When the determination means determines that the pulse width is not synchronized with the reference clock signals, it is determined that a receipt error occurs, and data restore operation by the data restore means is stopped.
Preferably, the edge invalidation means includes pulse width detection means. The pulse width detection means detects the pulse width between the present pulse edge and the next pulse edge, when the pulse width between the preceding pulse edge and the present pulse edge is shorter than one cycle of the reference signals. If the pulse width detected by the pulse width detection means is equal to or shorter than a predetermined threshold which is shorter than one cycle of the reference signals, the invalidation means invalidates the present pulse edge and the next pulse edge so that the two pulse edges are not inputted to the determination means. If the pulse width detected by the pulse width detection means is longer than the predetermined threshold, it is determined that a receipt error occurs, and data restore operation by the data restore means is stopped.